Various devices have been developed to improve the administration of intravenous (IV) fluids to patients in a controlled and accurate manner. One such device, is the peristaltic infusion pump. The peristaltic pump mechanism typically operates a series of fingers or rollers which deform and occlude a resiliently deformable IV drip tube at multiple points sequentially along the tube's length. These occlusions form a wave like motion which forces the IV fluid under positive pressure along the tube. The flow rate and hence dosage of the liquid is controlled by adjusting the speed of the pumping mechanism.
The finger type peristaltic pump typically comprises a motor, a set of gears, a cam shaft, a plurality of cams and a plurality of cam following fingers as per the assignee's invention described in U.S. Pat. No. 5,741,121. The finger type peristaltic pump is especially well suited for medical applications as only the tube comes into contact with the fluid, eliminating the risk of the pump contaminating the fluid, or the fluid contaminating the pump. Another advantage for medical applications is that fragile blood cells are not damaged by the pump.
An infusion pump occasionally needs to operate on Direct Current (DC), and not on Alternating Current (AC), such as for example in emergencies or where a battery needs to be used where mains power is not accessible or available. In these situations, power consumption is vital.
The fingers or cam followers of the finger type peristaltic pump are typically driven by rotating cams with varying profiles. In order to maintain a constant and accurate fluid flow and to reduce power consumption as well as to reduce audible noise and vibration at the motor, it is desirable that the cams on the shaft, rotate smoothly at a controlled speed. As the cam followers contact and apply a force to the cams, the cams in turn apply a tangential force to the shaft which produces a rotational torque tending either to aid or to impede the rotation of the shaft. This torque therefore causes variations in load presented to the motor. Such load changes affect the power consumption of the motor and may in turn adversely impact audible noise and the accuracy with which the pump delivers fluid.
NATWICK (U.S. Pat. Nos. 5,357,827 and 5,488,769) teaches an infusion pump incorporating a torque compensating cam. The purpose of the torque compensating cam in NATWICK, is however to aid in keeping the motor of the pump running at a constant rate, and not to aid power consumption.
Many high speed devices and machines make use of drive shafts to run various power take-off devices. The drive shafts of these machines experience variable torque at each torque load point where a power take-off device is connected to the shaft. In high speed engines, such as for example internal combustion engines, variable torque can cause angular twisting of the drive shaft which leads to fatigue and wear on both the drive shaft and the driven components. It is well known to use compensation cams in such applications to reduce the torque applied to the engine shaft (see U.S. Pat. Nos. 5,040,500 and 5,544,537). The purpose of using compensation cams in such applications is to prolong the life of the cam shaft by reducing vibration and wear, and not to reduce power consumption.